The present invention relates to a closed-loop mixture control system for a vehicle's internal combustion engine.
Closed-loop mixture control operation is based upon a feedback signal derived from an exhaust gas sensor which senses the concentration of a predetermined constituent gas of the exhaust emissions so as to represent air-fuel ratio within the exhaust system of the engine. This sensor is designed to produce a unipolar signal which has a rapid transition in amplitude at stoichiometry between zero voltage level to a certain positive potential. However, there is a certain type of exhaust gas sensors which exhibit bipolar output characteristics. Since the signal of the gas sensor output must be processed in later stages to develop a suitable control signal, the bipolar characteristic of the sensor signal would necessitate the use of two storage batteries, one of which is grounded at its negative potential terminal and the other of which is grounded at its positive potential terminal, if such exhaust gas sensors are employed in a motor vehicle. Since the automotive vehicle is usually equipped with a single source of DC voltage, the employment of the additional DC source is disadvantageous.